In general, the vertical structures of semiconductor devices have become more complex due to the increased integration density of the devices. As a result, the etching and cleaning processes which are employed in forming micro-patterns in the semiconductor devices have become more difficult. In particular, high energy ions are used to etch substrates, typically silicon substrates, forming isolation trenches of capacitors and other structures. As a result, a damaged silicon layer is usually generated. This is disadvantageous in that the presence of the damaged layer leads to potential performance degradation of the semiconductor device. Removal of the damaged layer is therefore important.
FIGS. 1 through 3 illustrate conventional methods for removing damaged silicon layers in semiconductor devices. FIG. 1 shows the formation of a first oxide layer 12, typically silicon dioxide formed by thermal oxidation, and a second oxide layer 14, typically by chemical vapor deposition (CVD) on a semiconductor substrate 10. The substrate 10 is then etched to form a trench 16. The trench 16 is typically formed by anisotropically etching in sequence, the second oxide layer 14, the first oxide layer 12, and the silicon substrate 10. High energy ions are used in the anisotropic etching. At this time, a silicon layer damaged by the high energy ions is formed under the trench 16.
FIG. 2 illustrates the conventional step of removing the damaged silicon layer, denoted by 17. More specifically, the damaged layer 17 is typically removed by a solution containing nitric acid (HNO.sub.3) and hydrofluoric acid (HF). Employing such a solution may present disadvantages. For example, the rate of etching silicon with the solution (several .mu.m/min) is extremely high in view of the desired etching thickness of the silicon (tens of nm). As a result, the use of the solution may be impractical since conventional contact times remove excessive amounts of silicon. An attempt to slow the etching rate of the solution by utilizing a diluted solution typically does not solve the above problem since the etching distribution of diluted solution is wide, ranging from several nm to tens of nm. Moreover, because the etching rate of the silicon dioxide layer is about 1/10th that of the silicon substrate, an undercut represented by the reference symbol "A" in FIG. 2 is often formed. As a result, subsequent filling of the trench is made difficult. Other problems are caused by the presence of the undercut. In FIG. 3, for example, a third oxide layer 18 and a polysilicon layer 30 are formed to fill the trench. Nonetheless, because of the undercut, a cavity 21 is disadvantageously generated inside the trench.
Other solutions have been used in attempting to remove the damaged silicon layer, namely diluted hydrofluoric acid solutions. Use of these solutions, however, is not desirable, since additional oxidation processes and cost are usually required in order to remove the damaged silicon layer.
In view of the above, there is a need in the art to provide solutions for etching semiconductor devices which exhibit more adjustable etching selectivity of the substrate relative to the oxide layer so as to maximize etching efficiency.
There is also a need in the art to provide methods for etching semiconductor devices with the solutions which are relatively simple.